In a pump of this type, there is an oil seal between the compression lobes and the compression chambers, and the pump is said to be "dry". However, the housing in which the gearing is situated contains oil to lubricate the gearing, and although the housing is separated from the nearest compression chamber by a lip seal around each shaft, the resulting sealing is insufficient when such a pump is used as a primary pump in which the compression chambers should be absolutely free from oil.
Depending on the suction pressure of the pump, the pressure in the compression chamber nearest to the housing, i.e. the chamber constituting the high pressure stage, may be greater than the pressure in the housing, giving rise to the danger that the lip of the lip seal will be raised.
In order to remedy this problem, it would be possible to balance the pressure on either side of the seal by providing a communication duct between the housing and the compression chamber, however that would run the risk of facilitating a flow of oil vapor via said duct from the housing into the compressive chamber, thereby leading to traces of oil condensation in the compression chamber.
The use of a seal having two lips mounted in opposite directions is also to be avoided, since the lip adjacent to the compression chamber cannot be lubricated and will therefore wear, giving rise to a very short lifetime.
In addition, the compression chamber adjacent to the housing runs very hot because of the gas compression, and this high temperature is transmitted to the oil, thereby firstly reducing its viscosity thus increasing leakage, and secondly increasing oil vaporization and making vapor trapping more difficult.
Finally, if the gases being pumped are corrosive, they can damage the seal rapidly and give rise to leaks.
The object of the present invention is to remedy these drawbacks.